In principle high quality NQR spectra can be obtained with commercial NMR spectrometers. In fact, most experiments involving NQR are done in solid state NMR. However, there are several drawbacks limiting the use of such devices when searching for yet unknown spectral peaks :
Our aim was to develop a low-cost wideband zero-field NQR coil without the need for tuning and matching. We developed prototypes for a spectral range between 75 and 145 MHz. The price to be paid for a wideband design is a comparatively low Q-factor and thus a worse SNR than that of a resonator. However, at VHF frequencies the NQR signals are frequently strong enough so that highest possible SNR may not be essential. Fast screening of several substances for only locating the NQR peaks thus is also possible with a lower Q-factor .
At the institute of Medical Engineering a dedicated zero-field NQR spectrometer has been developed which is equipped with special ultra-wideband probeheads.
Fig. 1 shows the system overview of the wideband NQR spectrometer (50 - 150 MHz) developed at the Institute of Medical Engineering. As spectrometer unit and RF generator a network analyzer was used. The remaining parts essentially constitute a custom-built RF unit for NMR including a specially designed no-tune/no match ultra-wideband coil .
Figure 2 shows the current implementation of the system during operation.
The probehead is a TX/RX coil and forms part of a reactance network which approximates an exponential transmission line. The network is terminated by a low resistance which dissipates most of the RF energy fed into the network by the power amplifier. In this way the input reflexion coefficient remains low over a wide bandwidth of several tens of MHz without the need for tuning and matching. This concept allows for the construction of robust and simple probeheads. The price paid for the simplicity is a lower SNR thanthat found in resonating circuits. However, for sufficiently concentrated samples with volumes between 400 and 1000 microliters the signal strength is usually high enough for a fast scanning of the NQR spectrum. Fig. 3 shows an example spectrum of the 79Br isotope in a sample of ZnBr2 measured with the spectrometer. See  for further reading.
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